As the 12V is ELV then you seem to be able to use any of Brown, Black, Red, Orange, Yellow, Violet, Grey, White, Pink or Turquoise for L according to table 51.

So if neither of the distributed 12V wires is earthed (i.e. the 12V system is unearthed or earthed to say a mid-point that isn't distributed), then you've got two non-neutral conductors, so it seems to me you can use any of that long list - including red/white or red/black.
- Andy.

Whilst in theory voltage drop may be calculated from published data, in the real world the exact load and the actual circuit length are not allways known.

I usually proceed as follows.

General purpose lighting 2.5mm cable, 10 amp fuse.
Small power 4.0mm cable, 16 amp fuse.
For short runs within a normal size dwelling, if in doubt, one size larger.

A 10 amp lighting circuit would be designed so as not to be regularly loaded to more than 5 amps, say 5 lamps each 11 watt CFL or 20 lamps each 3 watt LED.
If the lamp wattage to be used is unknown or likely to be altered then I allow only 4 lamps per circuit.

Small power sockets would normally be USA type mains sockets, readily available but not normally used for mains voltage in the UK.

For larger loads or long cable runs then I would calculate the voltage drop and size the cable accordingly, allowing between 3% and 10% voltage drop according to the type of load, and likely hours of use.

In view of the preference for non standard colours and the need for relatively large cables by domestic standards, I would normally use PVC singles in insulating conduit.

Can I suggest that the cable you use is proper ELV as in car/boat stuff? 6mm elv cable is about the same diameter as 2.5mm 500V stuff
Just finished wiring a boat and was surprised how small the overall diameter was. Obviously, usual rules apply on compartments etc.

I would normally use standard mains rated wire or cable for ELV installations.
IME the ELV stuff is less well made and often as expensive.

Whilst I would prefer to keep mains voltage and ELV in different conduits etc, this is sometimes not convieient, and if everything is insulated for the highest voltage present then that simplifies matters.

Mains rated PVC singles are also available in a wide range of colours which can be handy.